The present invention relates generally to detection systems, and more particularly, to a method and apparatus for imaging gamma rays.
The various organs and tissues of the human body fall prey to a myriad of different afflictions. For example, each year in the United States alone, approximately 180,000 women are diagnosed with breast cancer and 46,000 women die of this disease. In all, 10 to 11 percent of all women can expect to be affected by breast cancer at some time during their lives. The causes of most breast cancers are not yet understood. Screening and early diagnosis are currently the most effective ways to reduce mortality from this disease.
Currently mammography is the most effective means of detecting non-palpable breast cancer. However, mammography cannot determine whether a lesion is benign or cancerous, typically one or more biopsies must be performed per lesion. Unfortunately the biopsy operation itself is a very traumatic and costly operation that often results in some degree of disfigurement. Therefore it is important to improve the specificity of mammography thereby reducing errors, patient trauma, and disfiguration from unnecessary biopsies. It is also important to reduce health care costs by decreasing the number of unnecessary biopsies. For example, to detect 100,000 non-palpable cancers, approximately 500,000 biopsies must be performed at a cost of about $5,000 per biopsy, yielding a total cost of approximately 2.5 billion dollars. Therefore a reduction of 50 percent would save about 1.25 billion dollars per year.
Palpable mass abnormalities of the breast are often difficult to evaluate mammographically. This is especially true for patients with dense or dysplastic breasts (approximately 35 percent of women over 50 and 70 percent of women under 50) or those patients that exhibit signs of a fibrocystic change, for example due to radiation therapy. For example, invasive lobular carcinoma in dense breasts can attain a size of several centimeters and yet still show no mammographically detectable signs. Furthermore, about 50 percent of all preinvasive cancers do not show mammographically significant calcifications, thus decreasing the chances of detecting the malignant tumors.
Lastly, due to the interpretational limitations of mammography, many high risk patients (i.e., patients with a family history of breast cancer, patients with prior histologic evidence of cellular atypia, patients with a prior history of breast cancer who have undergone lumpectomy and radiation therapy) may be forced to rely on random tissue biopsies performed on suspicious areas. Unfortunately this technique typically results in a high nonmalignant-to-malignant biopsy ratio.
A relatively new scientific tool that has allowed scientists and physicians to address problems in physiology and biochemistry in the human body with low risk is emission computed tomography (ECT). ECT systems are mainly used for the detection and imaging of the radiation produced by radiotracers and radiopharmaceuticals. For example, by administering biologically active radiopharmaceuticals into a patient it is possible to image organ functions in real time.
The two major instruments presently used for ECT are Single Photon Emission Computed Tomography (SPECT) and Positron Emission Tomography (PET). These instruments have been used to study a variety of different organs and conditions including cerebral glucose consumption, protein synthesis evaluation, cerebral blood flow and receptor distribution imaging, oxygen utilization, stroke, heart, lung, epilepsy, breast cancer, dementia, oncology, pharmacokinetics, psychiatric disorders, and radio labelled antibody and cardiac studies. Since the SPECT and PET instruments use different types of radiotracers, the metabolic activities imaged are mostly different leading these two instruments to complement rather than compete with each other. The SPECT detectors have proven especially useful for heart and brain imaging.
SPECT dates back from the early 1960s, when the first transverse section tomographs were presented by Kuhl and Edwards (1963) using a rectilinear scanner and analog back-projection methods. With the availability of computer systems and the impetus of computer-assisted tomography using transmitted x-rays, nuclear medicine instruments were modified, and a number of mathematical approaches to tomographic reconstruction were developed in the early 1970s. Rotating Anger cameras and advances in computers opened the way to three-dimensional SPECT systems. Recently interest in SPECT increased as mathematical reconstruction techniques improved. They allowed for attenuation compensation, scattered radiation correction and the availability of new radiopharmaceuticals with higher uptake in the brain or other organs. The major limiting factors for the SPECT systems presently are the sensitivities (≈10 Cts sxe2x88x921 xcexcCixe2x88x921 point and ≈1,000 Cts sxe2x88x921 cmxe2x88x921 volume), resolution (7 to 12 mm FWHM), size, and cost.
Present SPECT systems mainly use the rotating Anger camera. Many different variations of the Anger camera and other smaller size rotating single or dual instruments have been designed and used. Most of the commercial instruments use NaI(Tl), CsI(Tl), CsF, BaF2, BGO and other related crystal detectors. The majority of the commercial instruments use the Anger cameras made of NaI(Tl) crystals. All commercial SPECT instruments use collimators for determination of the direction of the incident gamma rays. The main types are parallel and converging collimators. The converging fan or cone beam collimators produce higher sensitivity but increase the complexity of the data analysis. Pinhole and slit collimators are also used. The collimators for high resolution systems eliminate about 99.9 percent of the incident gamma rays. A typical collimator hole has an area of about 1 square millimeter and a length of 1.9 centimeters. Increasing collimator resolution decreases sensitivity and vice versa. Collimators made of high atomic number materials such as lead which also produce considerable amounts of scattered gamma rays on the inside surface of the collimator, thereby increasing the scattered photon background.
Anger cameras are normally rotated on a gantry around the patient for about 20 minutes to acquire sufficient data for a reasonable image. The spatial resolutions are limited to about 7 to 12 millimeters although spatial resolutions are expected to reach 6 millimeters in the near future. The best energy resolution at gamma ray energies is about 10 percent, limiting the ability of Anger cameras to discriminate scattered photon background. Commercially available SPECT systems include ADAC ARC, GE Starcam, Elscint APEX, Trionix Triad, Digital Scintigraphics ASPECT and University of Michigan SPRINT II.
From the foregoing it is apparent that an improved gamma ray imaging system is desired.
The present invention provides a high sensitivity, high spatial resolution, and electronically collimated single photon emission computed tomography (SPECT) system. Its primary sensitivity is in the range of 81 keV to 511 keV although it can be used to detect higher energies of up to a few MeV by increasing the detector thickness for both the hodoscope and the calorimeter. Both the direction and energy of the incident gamma ray photons is measured with high resolution. The method of determination of the photon direction eliminates the need for a mechanical collimator and the energy measurement discriminates against the scattered photon background.
The disclosed system is constructed from position sensitive, double sided silicon strips with a strip pitch of approximately 1 millimeter or silicon microstrips with a strip pitch much less than a millimeter. Preferably the system uses the silicon strip detectors. These detectors, varying in thickness from 150 micrometers to 2 millimeters, can produce the x and y coordinates of a photon interaction in a single wafer.
One embodiment of the system uses multiple planes of double sided silicon strip detectors with about 1 millimeter pixel size and a thickness of 100 micrometers to 5 millimeters. The planes are separated by a distance of between 0.2 and 2 centimeters, depending on the pixel size and the required angular resolution. The smallest possible separation is always preferred to keep the depth of the detector small without sacrificing spatial resolution. The incident gamma ray Compton scatters in one of the detector planes, the dominant process for photons with at least 50 keV energies in silicon strip detectors. The energy of the scattered electron in this detector plane is measured. The scattered gamma ray with reduced energy can be absorbed in the calorimeter or in an another detector plane through the photoelectric effect or undergo multiple Compton scatters followed by a photoelectric effect. The energies of these subsequent interactions are also measured. If the scattered gamma ray photon is completely absorbed, the sum of the two energies gives the energy of the incident photon and the individual energies and direction of the scattered photon give the scatter angle of the incident gamma ray. Thus the gamma rays emitted from a radionuclide can be imaged without need for a collimator.
The scattered gamma ray photons can make a second Compton scatter and then escape without further interaction. Also the photons already scattered inside the patient will deposit lower total energy. These events will produce a tail at lower energies in the energy spectrum. Such events can be discriminated effectively because the total energy detected is smaller than the known incident gamma ray energy. However, a high sensitivity mode may be applied with reduced angular resolution by adding the missing energy to the energy measured at the second scatter. This will dramatically increase the sensitivity but reduce angular resolution somewhat and will not allow the discrimination of the scattered photon background.
A calorimeter surrounding the silicon strip detector hodoscope absorbs the Compton scattered photons. The calorimeter can be fabricated from a plane of silicon a few millimeters thick, CdZnTe strip and/or detectors, or CsI(Tl) crystals viewed by a photodiode. The calorimeter can also be used as a second scatterer and/or a missing energy detector.
The double Compton scatter measurement determines the direction of the incident gamma ray to a cone with a half angle equal to the scatter angle. This type of measurement is new in nuclear medicine and requires special data analysis software. The data analysis can be carried out by cone interaction, Maximum Likelihood or Maximum Entropy techniques. These are iterative techniques and require long computation times. A new direct data analysis and imaging technique, Direct Linear Algebraic Deconvolution (DLAD) method, can also be applied for real time imaging.
In use, the present system utilizes the higher uptake of certain radiopharmaceuticals by the organ or tissue of interest, thereby allowing the selected organ/tissue to be imaged. For example, malignant tissues preferentially absorb Tc-99m SestaMIBI and Tl-201 chloride as compared to benign masses (except for some highly cellular adenomas). Therefore, these radiopharmaceuticals can be used to help diagnose and differentiate tumors from benign growths, for example in a scintimammography system for breast cancer detection and diagnosis. Possible mechanisms for uptake of Tl-201 chloride into tumor cells include the action of the ATPase sodium-potassium transport system in the cell membrane which creates an intracellular concentration of potassium greater than the concentration in the extracellular space. Thallium may be significantly influenced by this system in tumors. In addition, a co-transport system has been identified which also is felt to be important in uptake of thallium by tumor cells.